1. Field of the Invention
The present invention relates to a method of determining a system compressibility value of a medical membrane pump drive as well as to a method of determining an air proportion and/or an air quantity in a medical fluid conveyed by a membrane pump.
Membrane pumps are frequently used in the field of medical engineering, and in particular in the field of dialysis technology, to pump medical fluids such as dialyzate or blood. In this respect, a membrane pump typically has a pump chamber closed by a membrane, wherein fluid can be pressed out of the pump chamber by pressing the membrane into the pump chamber and fluid can be sucked into the pump chamber by pulling the membrane out of the pump chamber. Fluid can hereby be pumped through the pump chamber in interaction with corresponding valves.
The pump chamber is in this respect mostly arranged in a disposable, for example in a pump cassette, that is coupled to a membrane pump drive. The membrane pump drive in this respect typically has a drive chamber that is likewise closed by a membrane. The pump chamber and the drive chamber are then coupled to one another such that the membrane of the pump chamber follows the movement of the membrane of the drive chamber.
With a piston membrane pump, the drive chamber is in this respect in hydraulic communication with a piston-in-cylinder unit. Hydraulic fluid can be pressed into or sucked out of the drive chamber by moving the piston, which has the consequence of a corresponding movement of the membrane of the drive chamber. Such an arrangement has the advantage that the pump pressure can be controlled by a corresponding control or regulation of the pressure in the hydraulic part. Furthermore, membrane pumps allow a simple balancing of the pumped fluids since the volume change of the pump chamber and thus the fluid displacement on a pump stroke corresponds to the volume change of the control chamber (with the opposite sign), wherein this can be determined exactly via the position of the piston of the piston-in-cylinder unit.
Error sources can, however, occur here. On the one hand, air collected in the pump chamber can have the result that the fluid quantity pumped through the pump chamber does not exactly correspond to the volume change of the drive chamber. Furthermore, due to a certain system compressibility of the membrane pump drive, the volume change of the control chamber can differ from the volume change caused by the movement of the piston of the piston-in-cylinder unit. In this respect, air that collects in the hydraulic fluid can in particular result in a certain compressibility of the hydraulic system. Furthermore, hoses that connect the piston-in-cylinder unit to the drive chamber can, for example, have a certain flexibility and therefore expand at an elevated pressure. A certain system compressibility that influences the values detected for the balancing can also occur with other drive mechanisms.
2. Description of the Related Art
A method is in this respect known from DE 19919572 A1 by which the air proportion can be determined in the fluid pumped through a pump chamber. For this purpose, the pump chamber is first filled by gravity and the starting pressure hereby resulting is measured. The cut-off valves of the pump chamber are thereupon closed so that a fluid volume enclosed therein results. With closed cut-off valves, the piston-in-cylinder unit is then actuated to act on the closed fluid volume with a predefined end pressure. The volume change of the fluid volume in the pump chamber accompanying this pressure change in this respect directly depends on the proportion of air in the enclosed fluid volume. The air proportion can therefore be determined with the aid of the volume change that is produced by the pressure difference and that is determined via the piston movement. In this respect, in DE 19919572 A1, the influence of the system compressibility of the membrane pump drive is taken into account by a fixedly predefined constant. However, the system compressibility can, for example, vary due to air collecting in the hydraulic fluid during the operation of the pump, which remains out of consideration in DE 19919572 A1.
A method is therefore known from DE 102011105824 B3 how the system compressibility of a membrane pump drive can be determined. In this respect, the system compressibility of the pump apparatus filled with gas is determined in that a start pressure and an end pressure are adjusted using a pressure sensor and the associated pump positions or pump sensor values are recorded. The spring constant which is set as equivalent to the system compressibility is determined on the basis of the value pairs.